I've just looked at my VM webmail page and there is a lot more than 1/3 of the page available for message headers so I'm guessing you have a low res screen.
It does have a banner ad and then some sort of VM toolbar before the actual gmail page. I'd suggest you use FireFox + Adblock Plus and use that to block both the banner ad and the VM toolbar which are essentially a waste of screen space.
Example (pages cropped horizontally + some personal info blanked out) ...
With ads (unchanged VM webmail page)
Ads removed (Ad and the VM toolbar frame both blocked)